It was previously proposed to the end of improving the maneuverability of a front and rear wheel steering vehicle in large steering angle situations to appropriately adjust the steering angle of the rear wheels in relation to the steering angle of the front wheels. Typically, the rear wheels are either kept in neutral position or steered slightly in an opposite phase to the front wheels when making a sharp turn with the aim of reducing the under-steer tendency in such a situation, and allowing the vehicle to make a brisk yaw movement. For instance, Japanese patent laid-open publication No. 59-77970 discloses a four-wheel steering system which comprises a steering unit for steering front wheels, a rear wheel steering unit for steering rear wheels, a vehicle speed sensor, and a rear wheel steering angle controller which increases the steering angle of the rear wheels in proportion to the increase in the steering angle of the front wheels when the front wheel steering angle is below a certain threshold value, and reduces the rate of increase of the rear wheel steering angle with the increase in the front wheel steering angle when this threshold value has been reached, this threshold value being progressively lowered as the vehicle speed decreases, so that the lateral acceleration acting on the rear wheels may be increased in a high speed range and reduced in a low speed range, and the maneuverability of the vehicle may be increased in both high and low speed ranges.
However, depending on the situation, if the rear wheels are steered in an opposite phase to the front wheels, the side force of the rear wheels may reach a saturation region, and the slip angle of the rear wheels may become excessive. In such a case, the yaw rate of the vehicle is strongly affected by the application of brake, the releasing of the accelerator pedal and the turning back of the steering wheel, and, in extreme cases, the behavior of the vehicle may become uncomfortably unstable. In particular, on a low frictional coefficient road surface typically found on a gravel road and an icy road, this tendency becomes pronounced.
It was also proposed in the copending U.S. patent application Ser. No. 08/122,615 filed Sep. 16, 1993, now U.S. Pat. No. 5,528,497; Ser. No. 08/161,139 filed Dec. 2, 1993, now U.S. Pat. No. 5,448,481; Ser. No. 08/218,134 filed Mar. 25, 1994, now U.S. Pat. No. 5,448,482; Ser. No. 08/218,135 filed Mar. 25, 1994, Ser. No. 08/218,116 filed Mar. 25, 1994, now U.S. Pat. No. 5,481,457; and Ser. No. 08/379,988 filed Jan. 27, 1995, now U.S. Pat. No. 5,489,004 to compute a deviation of the actual yaw rate of the vehicle from a standard yaw rate determined according to the steering angle of the steering wheel and the vehicle speed, and carry out a feedback control of the rear wheel steering angle for the purpose of optimizing the dynamic response of the vehicle, and removing the adverse influences of the external disturbances and the changes in the running conditions of the vehicle on the behavior of the vehicle. The contents of these copending applications are incorporated in the present application by reference.